This invention relates generally to beneficiation of ores, and more particularly to a process including the addition of promoter reagents prior to carbonate-silica flotation, to achieve enhanced reduction of the magnesium content of phosphate-containing ores by flotation.
Phosphoric acid for use as acid, fertilizer, or in producing other phosphorous-containing chemicals is prepared commercially from phosphate-bearing ores. The ores are mined and beneficiated to higher phosphate and lower impurity concentrations and then processed to phosphoric acid by any of several techniques. In a typical flotation process to beneficiate a slurry of the ore in water, the ore is prepared by comminuting and sizing to a range of fine-sized particles, flotation reagents are added to the slurry and the particles of the desired and undesired species are separated into a concentrate portion and a tailing portion, respectively, by flotation to achieve the initial stages of purification of the phosphate ore concentrate.
Flotation is based upon the principle that a slurry of ore in water may be chemically treated by flotation reagents so that air bubbles will selectively adhere to particles of a selected species, causing them to float to the surface. In a typical flotation process, a chemical collector is added to the slurry to activate the surface of those particles to be removed from the slurry, thereby rendering the particles hydrophobic, and air is bubbled through the slurry. The bubbles adhere to the activated particles, which float upwardly to the surface of the slurry with the bubbles. A foam containing the floated species is then skimmed from the upper surface of the slurry as an overflow from the flotation unit. A selective depressant may be added to the slurry with the collector to render other species hydrophilic so that bubbles will not adhere to those particles, and the depressed species will not float but instead remain in the slurry. The underflow, or non-floated species, is then removed, either continuously or in batch fashion, for further beneficiation or processing into acid.
The beneficiation steps required depend upon the impurity constituents of the ore, which in turn may vary regionally from mine to mine. As an example, several mining locations throughout the United States yield a phosphate ore containing a low level of magnesium impurity, and also commonly containing carbonate and silica impurities. A high level of magnesium impurity in the phosphate ore is particularly undesirable, as it produces undesirable precipitates in the final phosphoric acid products and increases caking in dry phosphate products. Therefore, low-magnesium phosphate ores are most desirable in preparing phosphoric acid, and historically these ores have been mined first wherever available. The high-magnesium phosphate ores have typically been avoided, discarded or used only sparingly by blending them with low-magnesium ores to achieve an overall product meeting desired magnesium limits. In recent years the low-magnesium ores have been exhausted or are approaching exhaustion in many locations, and it has become necessary to develop a process for beneficiating high-magnesium phosphate ores.
One process for beneficiating phosphate ores by flotation provides for an addition of a fatty acid collector and a fluosilicic acid phosphate depressant to a sized slurry of ore in water, a first flotation of carbonate impurities, addition of amines, and finally a second flotation of silica impurities. Magnesium is removed primarily with the carbonate impurities. This process achieves acceptable recovery rates and concentration of the phosphate values, but only relatively small rejection of the magnesium found in the feed ore. When this process is applied to feed ores having low magnesium contents, the minor reduction in magnesium during the beneficiation process may still produce an acceptable product for use in manufacturing phosphoric acid. However, when the feed material contains larger amounts of magnesium, above about 1.2 percent MgO, or when it is desired to achieve greater magnesium reductions in low-magnesium phosphate ores, this process is found to be unacceptable in that too large a magnesium content remains in the phosphate concentrate. Other processes also exist using different depressants and collectors. However, these produce similar results.
Accordingly, there has been a need for a process to recover phosphates and reject larger amounts of magnesium during beneficiation of phosphate ores containing magnesium impurities. The need is particularly acute as low-magnesium phosphate ores become unavailable, and high-magnesium phosphate ores must be used increasingly in the future. The present invention fulfills this need, and further provides related advantages.